Art of bonding of vacuum metallized coatings to metal substrates



United States Patent 3,281,262 ART OF BONDING 0F VACUUM METALLIZEDCOATINGS T0 METAL SUBSTRATES Robert M. Brick, Hinsdale, 111., assignorto Continental Can Company, Inc., New York, N.Y., a corporation of NewYork No Drawing. Filed Apr. 19, 1962, Ser. No. 188,864

7 Claims. (Cl. 117-50) This application is a continuation-in-part of mycopending application Serial No. 836,145, filed August 26, 1959, now US.Patent No. 3,123,493, issued March 3, 1964.

This invention relates to the preparation of coatings upon metalsubstrates by vacuum deposition of metal thereon from metal vapors, andis particularly connected with the production of securely bondedcoatings.

The method of this invention leads to the production of a compositeproduct having the advantage of a substrate of a physically strong,cheap and easily fabricated metal but with the substrate metal havingdisadvantages such as low corrosion or other chemical resistance, withan adherent thin coating of a metal which is nobler and more costly thanthe substrate but which acts to provide the composite with the requisitechemical propelties.

It has been proposed to vaporize a metal in vacuo, and effect depositionthereof on a metal sheet as a substrate. In practice, a heating isemployed to regularize the coating and improve the bonding effect. Withcertain pairs of metals for the substrate and the coating, the bondingis accomplished with an interalloying effect at the interface. Whensteel is coated with deposited aluminum vapor, for example, there isinteralloying of body-centered cubic iron and face-centered cubicaluminum. The two metals are so dissimilar electrochemically and ininteratomic spacing that such alloying is not a simple inter-v solutionbut rather a new phase or phases, i.e., intermetallic compounds, mustform. The iron-aluminum alloys such as FeAl and Fe Al are hard andbrittle so that while bonding can be accomplished in form effective ifno mechanical stresses are later to be exerted upon the coated sheet,the relatively thick alloy layers formed cause fractures along the alloyinterface when stresses are exerted, with resultant separation along theinterface between the iron base and the aluminum coating deposit.

According to the instant invention, an interalloying is produced for aninterface thickness of only a few atoms diameters, e.g. 3 to atomsdiameters, that is, it is not detectable by microscopic inspection or bycleavage between substrate and coating during fabrication. Therewith,controls are provided for the presentation of a completely clean andnon-gassing substrate surface, for the temperature at which coating isbeing effected, and competent to receive and effect a bond with thecoating metal which is resistant to severe chemical corrosion influenceswithout obstruction of the bond.

It has previously been proposed to clean the surface of a ferrous sheetin various ways. For example, in galvanizing, a steel strip is heated inair or oxygen to burn off oils, and then it is heated at 900 to 1,200degrees F. in hydrogen to reduce surface oxides back to elemental iron.Therewith, the reducing chamber contains hydrogen at super-atmosphericpressure, and the sheet is advanced therefrom to a zinc-coating bath,noting that the hydrogen is not then of great harm to the plating butmay assist in keeping the bath low in zinc oxide. When this proceduce ofcleaning by hydrogen is employed at super-atmospheric pressure, the sealbetween the chamber for such cleaning operation and the depositionchamber must be able to resist penetration of the vapors of the first orhigh pressure chamber into the second or high-vacuum deposition chamber:and any hydrogen within the substrate as it ice passes into thedeposition chamber can cause the formation of gases as a blanket at thesubstrate surface which interferes with deposition of the metal vaporthereon.

According to this invention, a hydrogen removal of surface oxygen iseffected by storing hydrogen within the body of the substrate, and laterwithdrawing this hydrogen through the surface under a temperaturecondition at which hydrogen is effective to reduce surface oxides: andincludes the employment of this hydrogen-withdrawing step as a means ofalso riding the substrate of chemisorbed oxygen, air adsorbed oxides,carbonates, hydroxides, and other volatile or evaporable impurities.Therewith the withdrawal of the hydrogen from the substrate by lowpressure or vacuum, prior to the advancement of the substrate into thevacuum deposition chamber, reduces the load upon the evacuationequipment for this deposition chamber and acts to prevent thedevelopment of a gas blanket at the surface of the substrate during thedeposition.

An object of the invention is the employment of a surface cleaningoperation for the purpose of introducing and storing hydrogen within themetal substrate, followed by a preliminary evacuation for withdrawingthe hydrogen through the surface of the substrate, with a heating duringthe withdrawal, and then by a deposition of metal upon the substrateafter cooling to a temperature lower than that of the hydrogenwithdrawal.

' first evacuation zone, the effecting of this cleaning and the removalof the stored hydrogen in the first evacuation zone by heating topromote the escape of the hydrogen, and effecting the deposition ofmetal from its vapor in a second evacuation Zone at a temperature belowthat in the first zone and at which the deposited metal has a lowdiffusion rate into the substrate effective to produce adhesion with theinterfacial alloy thickness restricted to not exceeding ten atomdiameters.

A further object is a procedure in which a primary atmospheric cleaningaccomplishes the storage of hydrogen within the substrate, followed bysubjection of the heated substrate to evacuation whereby to withdraw thehydrogen and effect thereby a removal of surface oxygen, and followed inturn by the-vacuum deposition of metal vapor upon the cleaned surfacefor a time and temperature period during which alloy formation isrestricted.

A further object is a process of preparing a metal coated metal sheet bycleaning the surface of the metal sheet and storing hydrogen in the bodyof the sheet, with a subsequent heating and evacuation to cause thestored hydrogen to evolve and be effective for a surface preparation ofthe sheet, and with a final vacuum deposition of metal there-on at alower temperature of insignificant diffusion effects between the metals.

Illustrative of the practice of this invention is:

Example 1 A succession of commercially clean sheets of blackplate steelabout 0.009 inch thick were treated with perchloroethylene to effectde-greasing and removal of rolling lubricant: this was by dippings or byspray. They were next given a 30 second dip in 5 percent hydrochloricacid solution, rinsed in water, given a 5 second dip in 1% sodiumhydroxide solution, rinsed in hot water, dried with a clean cloth toweland placed in a batch-type vacuum metallizing chamber connected to apump system which could maintain a vacuum of /2 micron pressure in thesealed chamber. The substrates thus prepared had 1 to 3 parts permillion of hydrogen in the surface layers thereof. The specimens wereabout 17 to 4 inches in size: each was bent into U-shape with 5 inchlegs which were in vertical position in the chamber, with a 7 inchhorizontal connection between them at the top. After evacuation of thechamber, electric current was caused to flow through the strip; and itstemperature was raised to 650 degrees F. in about 2 minutes. Thepressure when the heating of the specimen began was about A2 micron.During the heating, gas was evolved and the pressure rose to about onemicron with the pump system in continuous operation. It was notable thatthe gas evolution became more rapid at a specimen temperature of about600 degrees F., indicated by a pressure change to above one micron. Whenthe pressure began to drop again with the temperature then at 650degrees F., due to cessation'of gas evolution from the specimen, theheating current was cut off and the specimen allowed to cool: during thecooling to about 400 degrees F., no further gas evolution was noted, andthe pressure returned to the /2 micron capability of the pump system.

The chamber had vaporizing equipment in the form of two tungsten coilslocated about 5 inches below the horizontal portion of the specimen: thetungsten wires had a charge of aluminum. 'When the specimen reached thetemperature of 400 degrees F., the coils were heated conductively byelectricity so that the aluminum evaporated therefrom and was depositedon the specimen, largely on the inner surface thereof, that is, thesurfaces facing the coils. A coating thickness of 30 micro-inches wasobtained in about 20 seconds. The specimen was allowed to cool in thevacuum chamber to below 250 degrees F., e.g. to 200 degrees F.: the sealwas broken, and the coated specimen removed.

The sheets were subjected to fabrication, i.e. bending of the aluminumcoated product; and were employed as ends for containers. The containerswere filled with nonsterile luncheon meat by standard procedure.Containers with ends of the same substrate, which had been coated withaluminum after like cleaning but with the vacuum deposition at less than250 F., were likewise filled as controls. The cans were held at 40 F.,and specimens examined at 3 days, 7 days, 2 weeks and 3 weeks. The endscoated at temperatures below 250 F., e.g. at room temperature, showeddeterioration at 7 days and were progressively worse at 2 and 3 weeks,with 10 to 60% of the aluminum film adhering to the meat product andindicating separation of the coating from the base metal: sulfidestaining was from slight to moderate. Comparably the ends coated atabout 400 F. by the instant process showed excellent adhesion, nocleavage, and from a trace to very slight sulfide staining. When theends were removed, and washed with carbon tetrachloride for removingadherent fat so the specimens could be photographed, the remainder ofthe aluminum coating came off from some of the metal which had beencoated at below 250 F., and about 90% was detached from the otherspecimens of such metal with low temperature coatings. In comparison,the ends made from metal coated at 400 B, there was no detachment orappearance of inter-face corrosion: that is, the aluminum remained inplace as a sacrificial protection anodically for the underlying steelplate.

The adhesion of a coating to a substrate involves the interfacecondition. Specimens of the various materials were examined by electronmicroscope as to this condition. The steel substrate was dissolved awayby dilute nitric acid, thus leaving the coating film of aluminum; andthis film was picked up on an electron microscope grid-and examined bythe electron microscope at enlargement of 25,500 times. temperatureshowed a fine grain structure, with grain sizes of 0.1 to 0.25 micronfor one lot of specimens, with no sub-grain network apparent except fordislocation bands. Another lot had a grain size from 0.2 to 0.4

The specimens coated at room micron, with minor fine sub-grain networks:in this lot, there was difiiculty in removing the steel substrate, andthe electron microscope test revealed small dark spots from theincomplete removal. comparably, the specimens coated at 400 F. showed alarger grain size, from 0.4 to 1.0 micron; a great amount of sub-grainstructure was observed when the electron beam was first directed on thefilm, this structure shifting and disappearing during the observationdue to the energy effect of the beam thereon. The larger grain size ofthe product of Example 1, compared to those with coatings at temperaturebelow 250 F., demonstrates a difference of product by which the adhesionand absence of interface corrosion are exhibited under the criticalluncheon meat test.

The several types of specimens showed ability to be fabricated, exceptfor specimens where the vacuum deposi tion was made upon a substrateheated to 650 F. or above. In the latter, the diffusion was so rapidunder conditions otherwise as in Example 1, that the brittle interfacialalloy developed to a detectable thickness, and caused cleavage andseparation during fabrication. The ability of the material producedunder the present invention to withstand the corrosion test is of greatcommercial significance.

The procedure is applicable with metal substrates having a body-centeredcubic habit, for facilitating the bonding of later-applied metalcoatings thereto: for examples, bodies of carbon irons and steels andalloyed irons and steels having a major matrix component of ferriticstructure, including molybdenum and chromium steels. Likewise, thesubstrate need not be a sheet or Web of such metal because separatearticles, e.g., castings of the metal, can be treated as in Example 1 tostore hydrogen therein, and then heated in vacuum for effecting theremoval of surface oxygen-containing films by batch or conveyoroperations. Further, coatings other than aluminum, including titanium,chromium and cadmium, may be vacuum deposited upon such substratescleaned by this procedure, to attain adherent coatings.

In the practice accordingto Example '1, the rate of evolution of gasfrom the base plate was such that no blistering or increase of porositywas apparent, and the cleaned base plate received the deposited aluminumsmoothly. The deposition of this coating at 400 degrees F. resulted inno detectable amount of interface alloy: fabrication did not result incleavage, which occurs with films of the brittle alloy at thicknesses ofabout 10 atom diameters or over.

The invention is not restricted to the illustrative embodiment, and canbe practiced in many ways within the scope of the appended claims.

What is claimed is:

1. The method of preparing an aluminum-coated steel substrate, whichcomprises degreasing the surface layer of the metal substrate with acleaning liquid, treating the degreased surface for about 30 secondswith an acid solution having the approximate strength of a 5 weightpercent hydrochloric acid solution whereby hydrogen is stored in saidmetal surface in an amount of from 1 to 3 parts per million, thereaftertreating the surface layer of the steel substrate with an alkalinesolution having the approximate strength of a 1 weight percent sodiumhydroxide solution for about 5 seconds, rinsing the metal surfacecontaining the hydrogen with water, drying the metal surface, placingthe substrate in vacuo and therein heating'to a tem'perature of 500 to650 degrees F. Whereby to cause the stored hydrogen to come forth fromthe substrate until cvolution of hydrogen therefrom essentiallyceases,jcooling the substrate to a temperature below 500 degrees F.While preventing the access of. oxidizing gases thereto, and thereaftereffecting deposition-of aluminum on the surface'ofthe substrate undervacuum and at a sheet temperature of 250 to 500 degrees F., andtherepcrature below 200 degrees F. before a detectable interalloy filmhas been formed.

2. The method of claim 1, in which the substrate is heated in vacuo withconstant pumping for removal of evolved gases until initial pressureincrease ceases and the pressure falls essentially to the pressure priorto the heating.

3. The method of claim 1, in which the heating is to a temperature of600 to 650 F., the vacuum deposition is conducted with the substrate atabout 400 F., and the coated substrate is cooled to below 200 F. inabout 2 minutes.

4. The method of preparing a substrate having a metal surface of a majorportion of ferritic structure with a metal coating, which comprisesdegreasing the metal surface of the substrate with a cleaning liquid,treating the degreased surface for about 30 seconds with an acidsolution having the approximate strength of a 5 weight percenthydrochloric acid solution whereby hydrogen is stored in said metalsurface in an amount of from 1 to 3 parts per million, thereaftertreating the metal surface of the substrate with an alkaline solutionhaving the strength of a sodium hydroxide solution of about 1%concentration, rinsing the metal surface containing the hydrogen withwater, drying the metal surface, placing the substrate in vacuo andtherein heating to a temperature of 500 to 650 F. whereby to cause thestored hydrogen to come forth from the substrate until evolution ofhydrogen therefrom essentially ceases, cooling the substrate to atemperature below 500 F. while preventing the access of oxidizing gasesthereto, and thereafter effecting deposition of a metal selected fromthe group consisting of aluminum, titanium, chromium and cadmium on thesurface of the substrate under vacuum and at a sheet temperature of 250to 500 F., and thereafter cooling the coated substrate in vacuo to atemperature below 200 F. before a detectable inter-alloy film has beenformed.

5. The method of claim 4, in which the heating is to a temperature offrom 600 to 650 F., the vacuum deposition is conducted with thesubstrate at about 400 F., and the coated substrate is cooled to below200 F. in about 2 minutes.

6. The method of preparing a chromium-coated steel substrate, whichcomprises degreasing the surface layer of the steel substrate with acleaning liquid, treating the degreased surface with a hydrochloric acidsolution of about 5% concentration for about 30 seconds whereby hydrogenis stored in the surface layer of the steel substrate in an amount offrom 1 to 3 parts per million, thereafter treating the surface with analkaline solution having the approximate strength of a 1 weight percentsodium hydroxide solution for about 5 seconds, rinsing the metal surfacecontaining the hydrogen with water, drying the metal surface, placingthe substrate in vacuo and therein heating to a temperature of 500 to650 F. whereby to cause the stored hydrogen to come forth from thesubstrate until evolution of hydrogen therefrom essentially ceases,cooling the substrate to a temperature below 500 F. while preventing theaccess of oxidizing gases thereto, and thereafter effecting depositionof chromium on the surface of the substrate under vacuum and at a sheettemperature of 250 to 500 F., and thereafter cooling the coatedsubstrate in vacuo to a temperature below 200 F. before a detectableinter-alloy film has been formed.

7. The method of claim 6, in which the heating during the placing stepis to a temperature of 600 to 650 F., the vacuum deposit-ion isconducted with the substrate at about 400 F., and the coated substrateis cooled to below 200 F. in about 2 minutes.

References fired by the Examiner UNITED STATES PATENTS 2,434,291 1/1948Smith 117-127 2,469,537 5/1949 Wohrer 117-127 2,588,734 3/1952 Kolodney11750 2,856,312 10/1958 Now-ak et al. 11750 2,876,132 3/1959 Worden etal. 117-50 3,029,158 4/1962 Lee et a1. 1l750 3,043,715 7/1962 Clough117-107 3,123,493 3/1964 Brick 117-50 3,156,578 11/1964 Olive-r 11750OTHER REFERENCES The Corrosion and Oxidation of Metals, Ulich Evans, St.Martins Press Inc., New York (1960), pp. 393-426. Vacuum Deposition ofThin Films, Holland, John Wiley and Sons, Inc. (1956), pp. 328-329.

ALFRED L. LEAVITT, Primary Examiner.

RICHARD D. NEVIUS, Examiner.

A. H. ROSENSTEIN, Assistant Examiner.

1. THE METHOD OF PREPARING AN ALUMINUM-COATED STEEL SUBSTRATE, WHICHCOMPRISES DEGREASING THE SURFACE LAYER OF THE METAL SUBSTRATE WITH ACLEANING LIQUID, TREATING THE DEGREASED SURFACE FOR ABOUT 30 SECONDSWITH AN ACID SOLUTION HAVING THE APPROXIMATE STRENGTH OF A 5 WEIGHTPERCENT HYDROCHLORIC ACID SOLUTION WHEREBY HYDROGEN IS STORED IN SAIDMETAL SURFACE IN AN AMOUNT OF FROM 1 TO 3 PARTS PER MILLION, THEREAFTERTREATING THE SURFACE LAYER OF THE STEEL SUBSTRATE WITH AN ALKALINESOLUTION HAVING THE APPROXIMATE STRENGTH OF A 1 WEIGHT PERCENT SODIUMHYDROXIDE SOLUTION FOR ABOUT 5 SECONDS, RINSING THE METAL SURFACECONTAINING THE HYDROGEN WITH WATER, DRYING THE METAL SURFACE, PLACINGTHE SUBSTRATE IN VACUO AND THEREIN HEATING TO A TEMPERATURE OF 500 TO650 DEGREES F. WHEREBY TO CAUSE THE STORED HYDROGEN TO COME FORTH FROMTHE SUBSTRATE UNTIL EVOLUTION OF HYDROGEN THEREFROM ESSENTIALLY CEASES,COOLING THE SUBSTRATE TO A TEMPERATURE BELOW 500 DEGREES F. WHILEPREVENTING THE ACCESS OF OXIDIZING GASES THERETO, AND THEREAFTEREFFECTING DEPOSITION OF ALUMINUM ON THE SURFACE OF THE SUBSTRATE UNDERVACUUM AND AT A SHEET TEMPERATURE OF 250 TO 500 DEGREES F., ANDTHEREAFTER COOLING THE COATED SUBSTRATE IN VACUO TO A TEMPERATURE BELOW200 DEGREES F. BEFORE A DETECTABLE INTERALLOY FILM HAS BEEN FORMED.